Two-Dimensional Transition Metal Dichalcogenides and MetalOxide Hybrids for Gas SensingEunji Lee,† Young Soo Yoon,*,‡ and Dong-Joo Kim*,†

†Materials Research and Education Center, Auburn University, Auburn, Alabama 36849, United States‡Department of Chemical and Biological Engineering, Gachon University, Seongnam 13120, Republic of Korea

ABSTRACT: Two-dimensional (2D) nanomaterials have demon-strated great potential in the field of gas sensing due to their layeredstructures. Especially for 2D transition metal dichalcogenides (TMDs),inherent high surface areas and their unique semiconducting propertieswith tunable band gaps make them compelling for sensing appli-cations. In combination with the general benefits of 2D nanoma-terials, the incorporation of metal oxides into 2D TMDs is a recentapproach for improving the gas sensing performance of thesematerials by the synergistic effects of the hybridization. This Reviewaims to comprehend the sensing mechanisms and the synergisticeffects of various hybridizations of 2D TMDs and metal oxides. TheReview begins with the gas sensing mechanisms and synthesismethods of 2D TMDs. Achievements in recent research on 2DTMDs and their metal oxide hybrids for sensor applications are thencomprehensively compiled. To clearly understand the collective benefits of TMDs and metal oxide hybrids, the hybridizationeffects are discussed in three aspects: geometrical, electronic, and chemical effects.KEYWORDS: gas sensor, 2D materials, transition metal dichalcogenides, metal oxide, hybrids, nanocomposite, heterojunction

A significant upsurge in research interests in two-dimen-sional (2D) nanostructured materials has arisen, seeking to

utilize the extraordinary properties inherent to 2D materials inmultiple fields, including energy storage, electronics, and chem-ical sensing.1,2 A 2D nanostructure is defined as a single layer or afew atomic layers of nanocrystalline materials, which structurallyexist as nanosheets, nanomembranes, and nanoflakes.3 The 2Dnanostructure can generally be classified as either 2D allotropicelements or covalently bonded compounds.4 Originating fromtheir atomically thin layers, the 2D nanostructured materialsmanifest unique chemical and physical characteristics, whichshow great promise for sensing applications. Specifically, theatomic-scale thickness of a 2D nanomaterial magnifies thesurface-to-volume ratio, which exposes more active surfaces totarget gases. Van der Waals gaps between the characteristicsurface configuration of the layers and thickness-dependentphysical and chemical properties are distinct qualities that make2D nanomaterials outstanding candidates for gas sensors.5,6 Theability of 2D nanomaterials to identify gas analytes at room tem-perature offers the possibility of wearable gas sensors integratedon a flexible substrate.7

Among intrinsic 2D materials (graphene,8 transition metaldichalcogenides (TMDs),9 phosphorene,10 MXenes,11 etc.),extensively researched graphene and other carbon-basedmaterials are chemically inert, which means that surface func-tionalization with other molecules is required for gas sensingapplications. In contrast, 2D TMDs display versatile chemistryand tunable band gaps that are much more beneficial in the

design of practical gas sensing devices. Layer-dependent prop-erties in conjunction with surface structuremodifications such asexposure of particular edge sites, chemical doping, and ligandconjugation were investigated to improve sensing perform-ance of TMDs.12 However, the rapidly developing field of TMDresearch still needs to address challenges such as selectivity, gasresponse, response time, recovery time, and stability. To give afew examples, 2D TMDsmay suffer the same selectivity issues asmetal oxides by responding to more than one type of gas. Theresponse time of 2D TMDs sensors is sluggish, and recovery isoften incomplete. Over time, the surface of layered 2D TMDscan be partially covered by oxygen or moisture in an ambientatmosphere due to different kinetics of adsorption and desorp-tion, which may gradually degrade the sensing performance andlong-term stability.13 Therefore, more recent efforts to utilize 2DTMDs for chemical sensing has moved to exploration of hybrid-izing novel metals and/or metal oxides into TMDs with tailoredmorphology. Previous studies have successfully demonstratedthe improved gas sensing performance resulting from the hybrid-ization. Geometrical benefits can be expected to result fromintegrating metal oxides with various morphologies to facilitategas diffusion and absorption. Electrical and chemical function-alization of the sensing material can also be obtained bydesigning heterojunctions.

Received: September 21, 2018Accepted: October 1, 2018Published: October 1, 2018

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pubs.acs.org/acssensorsCite This: ACS Sens. 2018, 3, 2045−2060

© 2018 American Chemical Society 2045 DOI: 10.1021/acssensors.8b01077ACS Sens. 2018, 3, 2045−2060

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Although several reviews of 2D TMDs14,15 and 2D materialsfor gas sensing3,5,16−19 have been published recently, no reviewhas focused on TMDs andmetal oxide hybrids. Additionally, theimproved sensing performance resulting from the hybridizationhas been verified through recent results, but the mechanismsthat lead to these remarkable effects have not been clearly dis-cussed. Therefore, herein we comprehensively review the liter-ature of 2D TMDs and their metal oxide hybrids to understandthe fundamental mechanisms and current progress in the field ofgas detection. In this Review, we present the reported mecha-nisms of TMDs and metal oxides for the basic understanding ofeach material. The unique gas sensing characteristics of selected2D TMDs and their hybrids are then discussed. Lastly, thechallenges and future directions to develop hybrids-based sen-sors will be addressed with a full summary. This critical reviewwill provide great insight into the evolution of TMD materialsand their metal oxide hybrids to unravel the complications ofhybridization.

■ GAS SENSING MECHANISMTwo-Dimensional Transition Metal Dichalcogenides.

The basic principle of gas sensing in 2D TMDs is mainly basedon the charge-transfer processes between gas molecules and thesurface of sensing materials. Different from the conventionalmetal oxides, the 2Dmaterials act as a charge acceptor or donor,resulting in the resistance (or conductance) change of theoverall system. Once exposed to reactive gases, gas molecules areabsorbed on the surface of 2D materials by the electrostaticforce. The direction of electron charge transfer is determined bythe type of reactive gas, either reducing or oxidizing. The resis-tance of the sensing material is recovered up to its initial valuedue to desorption of gas molecules upon exposure to air. Theamount of resistance modulation is determined by the chargeaffinity of reactive gas to release or withdraw electrons. Using ap-type 2D material as an example, the resistance of the sensingmaterial usually increases under the exposure of reducing gases.Metal Oxides. The working principle of metal oxide sensors

has been extensively studied. The sensing mechanism of theconventional metal oxide sensors is related to adsorbed oxygenions on the oxide surface. The oxygen ions are formed by drawingelectrons from the conduction band of the metal oxide, anddifferent forms are possible depending on the operating temper-ature.20,21 Below 200 °C, electrons are attached to oxygenmolecules as shown in eq 1. Above 250 °C, the oxygenmoleculesare dissociated into oxygen ion atoms with electric charge bypulling electrons from metal oxide as shown in eqs 2 and 3.

+ ↔ [ ° ]− −eO (gas) O (adsorbed) below 200 C2 2 (1)

+ ↔ [ ° ]− −e12

O O (adsorbed) above 250 C2 (2)

+ ↔ [ ° ]− −e12

O 2 O (adsorbed) above 250 C22

(3)

The interaction between the oxygen ions and gas analytes canbe divided into two main categories depending on the majoritycarrier, electrons or holes. When the majority carriers areelectrons, the metal oxide is considered to be n-type, while themajority carriers are holes in the p-type metal oxide sensors.Using n-type metal oxides as an example, the electrons in theconduction band of the metal oxide are decreased due to thereactions of eqs 1−3, resulting in increased resistance at theoperating temperature. Once the target gas is introduced,

the electrons from the reducing gas are transferred to theconduction band of the metal oxide causing the resistance of thesensor to decrease. It should be noted that the opposite resis-tance change occurs with an oxidizing gas. The categorizedsensing behavior of n-type and p-type sensing materials toreducing and oxidizing gases is compared elsewhere.22 As anexample, NOx is considered as an oxidizing gas due to unpairedelectrons around the N atom, while NH3 acts as a reducing gasdue to its lone electron pair.5 Due to the dominant role of oxy-gen ions, the metal oxide sensors are typically operated atelevated temperatures.

■ GAS SENSING PROPERTIES OF 2D TRANSITIONMETAL DICHALCOGENIDES

TMDs are MX2-type inorganic compounds where M is atransition element such as Ti, Zr, Hf, V, Nb, Ta, Mo, W, Tc, orRe in groups IV, V, and VI of the periodic table, and X is anelement of the chalcogen species such as S, Se, or Te. Whenreduced to a single layer or a few layers, the TMDs have shownexcellent electric properties comparable to those of graphene.23

The 2D layered structure of TMDs consists of strong molecularintralayer bonds and weak van der Waals interlayer bonds. Theweak interlayer bonding allows exfoliation of TMDs down tosingle layers or multilayers by using mechanical exfoliation orelectrochemical intercalation.14 In general, the thickness oflayered TMDs is about 6−7 Å, and the length of M−M bond istypically between 3.15 and 4.03 Å, depending on the size of themetal and chalcogen atoms.24 Higher specific surface area andfunctionalities driven by such structure make 2D TMDspromising candidates for sensor applications.The electronic and magnetic characteristics of TMDs hinge

on the coordination number of the transition metal and itsd-electrons. The transition metal (M) atoms in TMDs have fourelectrons to fill the bonding states and chalcogen (X) atoms havean oxidation state of−2. Thus, the number of d orbital electronswill vary 0 to +6, which influences electronic properties. Theelectrical nature of TMDs can be tuned from semiconductors(MoS2, MoSe2, WS2, WSe2, ...) to metallic (NbTe2, TaTe2, ...,)or superconductors (NbS2, NbSe2, ...). The partially filledd-orbitals in transition metals provide metallic conductivity,while fully filled d-orbitals give semiconducting properties. Thesurface of layered TMDs is terminated by chalcogen atoms withlone-pair electrons. The absence of dangling bonds in TMDsstabilizes the material against ambient environment and giveshigh mobility in compliance with the substrates and metalcontacts. For instance, the mobility of molybdenum disulfide(MoS2) is about 700 cm

−1 V−1 s−1 on SiO2/Si substrate at roomtemperature.25 An important attribute of TMDs for gas sensingcan be a tunable band gap. Graphene, a popular 2D material, ismetallic with a zero band gap, but several TMDs such asmolybdenum disulfide (MoS2), tungsten disulfide (WS2), andtin disulfide (SnS2) have a direct and indirect band gap around1−2 eV due to their low intrinsic carrier concentration. Thefascinating electrical and optical properties of TMDs arise fromthe transition of an indirect band gap to a direct band gap as thethickness changes from bulk to monolayer due to quantumconfinement. Moreover, mechanically flexible and strong TMDscan be another advantage to extend fields of chemical sensors.It is reported that few-layeredMoS2 has a high Young’s modulusof 0.33 ± 0.07 TPa that outperforms the stainless steel andgraphene oxide.26

In the TMD class, disulfides such as MoS2, WS2, and SnS2 aregreat representatives of TMDs for chemical sensing. The crystal

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structure of molybdenum disulfide (MoS2) has been revealed formore than 5 decades, but its electrical and optical features havebeen investigated recently.27 In each layer of MoS2, Mo atomsare sandwiched between covalently bonded S atoms, with a layerheight of about 0.65 nm. The crystal structure of MoS2 with atrigonal prismatic arrangement manifests as hexagonal crystalstructure.28 Monolayers of MoS2 have a direct band gap of 1.8 eV,while bulk (>8 layers) MoS2 presents an indirect band gap of1.29 eV. Unique physical and chemical features of MoS2 for gassensors include a layer-dependent band gap, a high surface-to-volume ratio, and a high adsorption coefficient. Tungstendisulfide (WS2) is another TMD material. The structure of WS2is similar to MoS2, where tungsten layers are sandwiched inbetween two sulfur layers in a trigonal prismatic arrangement.It is reported that a single layer ofWS2 has a direct band gap witha theoretical value of 1.8−2.1 eV.15 For gas sensing applications,WS2 can be more attractive than MoS2 because WS2 has ambi-polar field-modulation behavior29 and high electron mobility atroom temperature up to 234 cm2/(V·s).30 In addition, thehigher photoluminescence (PL) emission efficiency of mono-layered WS2 suggests a higher quantum efficiency than MoS2crystals.31 Other types of 2D TMDs such as molybdenumdiselenide (MoSe2) and molybdenum ditelluride (MoTe2) havealso similar structural characteristics to MoS2 and WS2. Theatomic structure of single-layer MoSe2 is larger due to the largeratomic radii of the Se atoms compared to Sn. In a single layer ofMoSe2, the length of the Mo−Se bond is 2.53 Å, and the lengthof the Se−Se bond is 3.29 Å.32 The indirect band gap of bulkMoSe2 is about 1.1 eV, while the direct band gap of the singlelayer is about 1.55 eV.33 A recent experiment has revealed thatthe mobility of MoSe2 is∼50 cm2/(V·s) at room temperature.34

Interestingly, MoTe2 has larger bond length and lower bindingenergy than other TMDs, which drives its potential to improvedgas sensitivity.35 Although tin does not belong to transitionmetal category, tin disulfide (SnS2) formed a 2D layered phasewith hexagonal structure similar to TMDs.36 It is interesting tonote that larger electronegativity of SnS2 in comparison withother TMDs may suggest a high absorption ability correspond-ing to a high gas response. However, the relatively strongertemperature dependency of SnS2 on electronic band structuremay need consideration to determine optimal sensing temper-ature based on understanding absorption and recovery ofkinetics at different temperatures.37

Each 2D material presents different electrical features andsurface configurations, which influence their gas sensingproperties. Another important interpretation of gas sensingproperty in 2D TMDs can be conducted by understandingthe charge-transfer mechanism depending on their surface

configuration and electrical characteristics. The interactionsbetween sensing materials and gas molecules can be inferred bythe adsorption energy and the amount of charge transfer, whichare commonly evidenced by first-principles calculations basedon density functional theory (DFT). Such computationalsimulations were able to consider parameters includingadsorption positions and orientations of gas molecules on a2D material. Table 1 gives the results of the DFT simulations onthe most stable adsorption position and orientation. Absorptionenergy between 2D materials and gas molecules was calculatedby eq 4.42

= − [ + ]+E E E Ead (2D gas) (2D) (gas) (4)

where E(2D+gas) is the total energy of a supercell containing both a2D material and a gas molecule, E(2D) is the total energy of the2D sensing material, and E(gas) is the total energy of a supercellcontaining a gas molecule. Here, the negative value of Eadindicates an exothermic reaction, meaning thermodynamicallyfavorable reaction. The negative value of ΔQ means the direc-tion of charge transfer from gas molecules to sensing material orvice versa at the optimal adsorption configuration. The largevalue of adsorption energy can be interpreted as the level ofselective response to the gas molecule over other species.43 Theamount ofΔQ can be associated with the number of transferredelectrons between gas molecules and the sensing material.In addition, physisorption and chemisorption can be determinedfrom ΔQ value.Table 2 summarizes recent examples in gas sensing perform-

ance using the 2D TMD nanostructured materials. The per-formance of gas sensors is evaluated by gas response, selectivity,stability, response time, and recovery time. In general, the gasresponse is calculated by the ratio of resistance Rg/Ra or Ra/Rg tohave responding value greater than 1. Alternately, ΔR/Ra is alsoused to measure the resistance difference before and after gasinjection (ΔR = |Rg − Ra|), where Rg and Ra are the resistance ofgas sensors with target gas and the resistance of gas sensors inambient air, respectively. The response time is commonlydefined as the time spent to reach 90% of the gas response in thepresence of a target gas. Similarly, the recovery time is defined asthe time spent to return to 90% of initial status without gasmolecules.16 The sensing performance of the representativeTMDs including MoS2, WS2, MoSe2, MoTe2, and SnS2 waslisted in Table 2. Since most literature uses NH3 and/or NOx forevaluation of sensing properties, the results were compared interms of NH3 and/or NOx gas.

Molybdenum Disulfide (MoS2). The gas sensing mecha-nism based on the charge transfer in atomic layered MoS2 waselucidated using in situ PL by Cho et al.42 In situ PL spectra of

Table 1. Density Functional Theory Calculation of Adsorption Energy and Charge Transfer for 2D Materials

NH3 H2O NO2 NO CO O2 H2 ref

graphene Ead (eV) 0.031 0.047 0.067 0.029 0.014 − − 38ΔQ (e) 0.027 −0.025 −0.099 0.018 0.012 − −

MoS2 Ead (eV) −0.250 −0.234 −0.276 −0.195 −0.128 −0.106 −0.07 39ΔQ (e) −0.069 0.012 0.100 0.011 0.020 0.034 0.004

WS2 Ead (eV) − −0.18 − − −0.14 −0.11 − 40ΔQ (e) − 0.0048 − 0.0096 0.0078 0.0134 −

MoTe2 Ead (eV) 0.13 − 0.2 − − − − 41ΔQ (e) − − − − − − −

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theMoS2 were recorded in the presence of NO2 (p-type dopant)and NH3 gas (n-type dopant). The relatively low-energy A exci-ton was separated into a trion of A−/+ depending on the majoritycharge carrier and a neutral exciton of A0. For bare MoS2, thepositive trion peak (A+) of PL spectra was dominant, indicating ap-type characteristic of MoS2 on SiO2/p

+ Si substrate in Figure 1a.After NO2 exposure, the A

+ peak was increased and the A0 peakwas decreased, showing trade-off phenomena. A neutral exciton(A0) was converted to a quasi-particle (A+) owing to holeinjection from NO2 (electron extraction from MoS2) while theopposite tendency was observed when NH3 (electron injectionfrom NH3) was introduced. The in situ PL results experimen-tally confirmed the charge-transfer mechanism of atomic layeredMoS2. Theoretical studies have shown to support the charge-transfer mechanism. Yue et al.39 computed the adsorptionenergy and the amount of charge transfer between monolayerMoS2 and various gas molecules, O2, H2O, NH3, NO, NO2, and

CO using first-principles (Figure 1b). DFT calculations revealedthat these gas molecules are physically absorbed onto the MoS2with small charge transfer, acting to inject or extract electrons.During the physical absorption of the gas molecules, the bandstructure of MoS2 was not significantly changed, althoughseveral gases created adsorbate states in the band gap of MoS2.In theoretical aspect, TMDmaterials can be more favorable thangraphene for gas absorption. As listed in Table 1, Ead of MoS2 toall gas molecules exhibits a negative value, while graphene doespositive. The negative Ead of MoS2 suggests that gas moleculesare voluntarily absorbed on the surface of MoS2, with transfer-ring or withdrawing electrons with the amount of ΔQ.Much effort has been dedicated to enhancing gas sensing

performance of bareMoS2, by tailoring the number of layers andadjusting their alignment. The effect of the number of MoS2layers on the gas sensing performance was studied by He et al.45

The thickness of MoS2 layers was controlled by the volume of

Figure 1. Photoluminescence and charge density of MoS2 to n-type (NH3) and p-type (NO2) dopants. (a) In situ PL spectra recorded from the MoS2with NO2 and NH3 molecules. (b) Charge density difference plots to O2, H2O, NH3, NO, NO2, and CO on monolayer MoS2. The red (green)distribution corresponds to charge accumulation (depletion).

Table 2. Literature Study on Gas Sensing Performance of 2D Nanomaterial Gas Sensors Working at Room Temperature

material thickness analyte concentration gas response, ΔR/Ra (%) limit of detection response/recovery time (min) ref

MoS2 atomic layer NO2 5 ppm 15 − −/− 42NH3 1 5 ppm

atomic layer NO2 120 ppb 35 120 ppb −/− 44single layer NO2 1.2 ppm 6.1 2 ppb −/− 452 layers NO 2 ppm 80 0.8 ppm −/− 462 layers NO2 1 ppm 2.6 1 ppm 11.3/5.3 475 layers NO2 100 ppm 30 10 ppm −/− 920 nm NH3 2 ppm 0.01 300 ppb −/5 48

WS2 2−50 nm NH3 5 ppm 0.02 1.4 ppm −/− 49110 nm NH3 5 ppm 1.6 1 ppm −/− 501−44 layers NH3 250 ppm 2.6 50 ppm −/− 51multilayers (5 nm) NO2 5 ppm 68.4 0.1 ppm −/− 52

MoSe2 single layer NH3 300 ppm 300 50 ppm 2.5/9 32

MoTe2 few layers NO2 100 ppb 8 12 ppb −/− 41

SnS2 1−3 layers NH3 100 ppm 2.13a 20 ppm 53aIg/IO.

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MoS2 solution during a spin-coating process, and the conduc-tance of the prepared MoS2 was measured with and withoutNO2 and NH3 gases. Regardless of MoS2 layer thickness, theconductance ofMoS2 channels was increased whenNO2 gas wasinjected, showing p-type characteristics. In Figure 2a, the gasresponse to NO2 was reduced with increasing thickness of MoS2layers ranging from 2 to 18 nm since the surface-to-volumeratio of the MoS2 channel was decreased with increasingthickness. Late et al.9 prepared two- to five-layered MoS2 on aSiO2/Si substrate whose thickness is ranging from 1.4 to 3.3 nm(Figure 2b,c). The five-layer MoS2 film (3.3 nm) has shown thehighest gas response to both NO2 and NH3. The authors argueddifferent electronic structures such as the level of the conductionand valence bands of MoS2 responsible for better sensingproperties. However, a clear explanation on the specific sensingmechanism was not given to clarify the opposite tendency ofthickness dependence of gas response. Besides the number oflayers, the alignment of 2D layered MoS2 also has an impact onthe sensing properties. Cho et al.54 have demonstrated enhancedgas adsorption properties of MoS2 layers by vertically aligningthe layer direction. In their work, Mo seed layers were pre-deposited on SiO2 and converted to MoS2 film by sulfurizationduring the chemical vapor deposition (CVD) process. Thegrowth direction of CVD grown MoS2 was varied according tothe thickness of the Mo seed layer, and three different align-ments of MoS2 were prepared, horizontal, mixed, and verticalorientations. Their sensing properties ofMoS2 onNO2 andNH3showed the highest gas response from vertically aligned MoS2,and the lowest from the horizontal one (Figure 2d). The result isdue to higher absorption on edge sites of MoS2 than basal planesites, which arise from the high d-orbital electron density of Mo

(Figure 2e). Therefore, both geometric alignment of 2D layersand layer thickness should be considered to improve gas sensingproperties.

Tungsten Disulfide (WS2). Similar to MoS2, the factorssuch as the number of layers and alignment affect the gas sensingperformance of WS2. Qin et al.51 studied the relationshipbetween the number of WS2 layers and sensing performance.WS2 samples with different thicknesses were prepared by anexfoliation ranging from one layer to bulk. By increasing thenumber of layers, the gas response becomes larger due to moregas molecules inserted into their inner layers and interacted withthe two adjacent layers (Figure 3a). However, longer recoverytime was required in thicker layers since desorption becamemore difficult. The results can be explained from the first-principles calculations. The estimated binding energy betweenNH3 molecules and single layer of WS2 is−0.179 eV, implying aweak van der Waals interaction. On the other hand, the bindingenergy of NH3 molecules inside the interlayer is much strongerat−0.356 eV resulting from the top and bottom dual interactionof WS2 layers, which leads to longer recovery time. As studiedwith MoS2 on surface functionalization,54 WS2 edge-function-alized carbon nanofibers (CNFs) with multiple tubular pores(WS2@ MTCNFs) on gas sensing properties were investigatedby Cha et al.55 More edge of WS2 nanoflakes was functionalizedon the surface with CNFs structures, and higher specific surfacearea was obtained from MTCNFs, which improved gas sensingresponse (Figure 3b). However, the effect of surface area isdiminished as the concentration of NO2 decreases, suggesting arelatively minor influence of a porous structure at low levels ofconcentration. The selectivity toward NO2 also improvedagainst interference of NH3 and toluene (C7H8) (Figure 3c).

Figure 2. (a) Detection of 1.2 ppm of NO2 using a MoS2 thin film transistor on PET with different thickness of MoS2 thin film. (b) Comparative two-and five-layer MoS2 cyclic sensing performances with NH3 for 100, 200, 500, and 1000 ppm. (c) Comparative two- and five-layer MoS2 cyclic sensingperformances with NO2 for 100, 200, 500, and 1000 ppm. (d) Resistance response upon exposure to NO2 gas with concentrations ranging from0.1 to 100 ppm. (e) Schematic illustration of gas adsorption mechanism on edge sites and basal plane of MoS2.

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The enhanced selectivity ofWS2-edge-functionalized CNFsmayoriginate from the high density of WS2 edges that preferentiallyreact with NO2 gases. Apart from NO2 analyte, pure WS2showed selective response toward ammonia50 in Figure 3d.In addition to structural modulation of layered WS2, external

factor such as light illumination was investigated. Huo et al.56

reported that the charge transfer between the WS2 nanoflakesand the physical-adsorbed gas molecules is closely related to thephotoelectrical features of WS2 (Figure 3e). The source−draincurrent of the WS2 transistor device is slightly decreased in O2and largely increased in ethanol and NH3. Under the lightexposure, all gas molecules can be further absorbed due to photo-responsivity and external quantum efficiency. The dynamicresponse by light can be ascribed to two physical mechanisms;the photogeneration/recombination of electron−hole pairs andcharge transfer.57 Figure 3f displays the response of the WS2 to10 ppm of NH3 under different light illuminations, from infrared(IR) to ultraviolet (UV), at 40 °Cwith a relative humidity (RH)

of 30%. The response of the multistacked WS2 was enhancedunder the overall range of light due to the photosensitive naturesof the samples. Under the illumination of 365 nm light, theresponse of WS2 to NH3 was at the maximum of 3.4 (Rg/Ra)with the shortest response and recovery time of 252 and 648 scompared to the other range of the lights. This result suggestslight modulation should be a great option to improve the sensingperformance of TMDs.

Other TMDs: Molybdenum Diselenide (MoSe2) andMolybdenum Ditelluride (MoTe2). The first demonstrationof a gas sensor device based on single-layer MoSe2 was reportedby Late et al.32 Single-layer MoSe2 exfoliated from bulk MoSe2was fabricated on a SiO2 substrate using electron beam lithog-raphy. The gas response of the MoSe2 sensor was increased withhigher NH3 concentrations at room temperature9 (Figure 4a).The gas response value of MoTe2 was comparable with that ofMoS2, but the response time (2.5 min) and recovery time (9 min)were slightly shorter. The first principle calculations reported

Figure 4. (a) NH3 sensing response of single-layer MoSe2 as a function of gas concentration. (b) Real-time conductance change of p-typeMoTe2 FETsensor upon exposure to different concentrations of NO2 under different gate biases. (c) Real-time conductance changes of n-type MoTe2 FET sensorto different concentrations of NH3 under different gate biases.

Figure 3. (a) Response−recovery curves of thin WS2 nanosheets and bulk WS2 interaction with 250 ppm of NH3 at room temperature. (b) Dynamicresponse transients of CNFs, WS2 @ CNFs, and WS2 @MTCNFs to NO2 in the concentration range of 1−4 ppm at room temperature. (c) Selectiveproperty of CNFs and WS2 @ MTCNFs toward 5 ppm of NO2, NH3, and C7H8. (d) The selectivity of the WS2 nanoflake-based sensor to differentgases at RH = 60%. (e) The extracted dark current and photocurrent under different gas atmospheres. (f) Response curves of the WS2-basedchemiresistive sensors under different light illuminations driven by DC power to 10 ppm of NH3.

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that MoSe2 can be sensitive to NO2 due to its large adsorptionenergy.58 Such gas adsorption also strongly influences theelectronic characteristics of monolayer MoSe2 by inducingimpurity levels at near Fermi energy. For example, NO2 has alarger absorption energy than NO and SO2, which guides thatMoSe2 is more sensitive to NO2.Gas sensors based on MoTe2 were investigated by Feng

et al.54 Few-layered MoTe2 flakes (6.5 nm) were exfoliated froma semiconducting 2H-MoTe2 bulk and transferred onto SiO2/n+-doped Si substrates. Figure 4b,c shows the conductancechange of p-type MoTe2 field effect transistor (FET) upon theexposure of NO2 and NH3 at different concentrations. Inter-estingly, varying gate biases on the MoTe2 FET was conductedto investigate recovery kinetics. The MoTe2 sensor displayed anexcellent recovery time of below 10 min, in comparison tographene and MoS2 whose recovery times were longer than 1 hat room temperature.59,60 At zero bias, the recovery time in NH3

was shorter than that in NO2 because of the lower bindingenergy of NH3 on MoTe2 surface. This result showed goodagreement with DFT calculations where the binding energy ofMoTe2 is 0.2 eV to NO2 and 0.13 eV to NH3. The smallerbinding energies of MoTe2 to NH3 and NO2 enable efficientdesorption of gas molecules on the surface of MoTe2 at lowthermal energy. These experimental and theoretical resultssupport that bothMoSe2 andMoTe2 can be great candidates forchemical sensing applications.Metal Dichalcogenide: Tin Disulfide (SnS2). The

physisorption-based sensing properties of SnS2 were reportedby Ou et al.37 Few-layered 2D SnS2 flakes were synthesized via afacile wet chemical route with the average thickness of 6 nm.Figure 5a displays that the response factor (Rg/Ra) and gasresponse time to 10 ppm of NO2 at an elevated temperature of80 to 160 °C. The adsorption of NO2 gas molecules onto theSnS2 surface was promoted by increasing the operating tem-perature to 120 °C, and the highest gas response value of 36 wasobserved at 120 °C. The extrapolated detection limit of SnS2toward NO2 determined from 0.6 to 10 ppm was estimated to be20−30 ppb. High response was only obtained from NO2, indi-cating a strong selectivity to NO2 over H2, CH4, CO2, and H2S(Figure 5b). In the DFT calculation, the expected physisorptionenergies of SnS2 flakes to NO2, NH3, H2S, CH4, CO2, H2 and O2in a 3×3×1 supercell are −0.367, −0.215, −0.199, −0.191,−0.053, 0.182, and 1.430 eV, respectively. These values describethat NO2 has the greatest adsorption energy, indicating betterselectivity of SnS2 sensor toward NO2.

The sensing performance of layered SnS2 can be tuned bycontrolling defect structure. Qin et al.53 reported that theexfoliated SnS2 sensor exhibits excellent ammonia sensing whilethe bulk SnS2 sensor does not respond. Such observation can beexplained in terms of the carrier mobility, high surface-to-volume ratio, and excellent shielding noise natures of 2D SnS2material.61,62 The sulfur vacancies of the exfoliated SnS2 serve asthe major gas absorption sites with high binding energy becauseof their high activity, catalysis and dissociation characteristics.The vacancy assisted gas absorption was proven through theDFT calculations.63 Although the formation energy of theS-vacancy system is not thermodynamically favorable, the for-mation of S-vacancies is unavoidable during the synthesis pro-cess, which can enrich the gas adsorption on the host monolayer.The simulation results also help to predict types of gas adsorp-tion. H2S and COmolecules were physisorbed on S-vacancies inthe SnS2 monolayer, whereas NH3, NO, and NO2 moleculeswere chemisorbed via strong covalent bonds. The effect ofbackground oxygen content on the gas sensing performance alsosupports the role of S-vacancies.64 Figure 5c describes theinfluence of ambient oxygen concentration on the response ofthe SnS2 nanoflowers at 200 °C. The increased sensing responseat higher oxygen concentration is explained in terms of thecatalytic redox reaction of NH3. To confirm the experimentalresult, the molecule−surface adsorption energy of the SnS2nanoflowers to NH3 was calculated with considering oxygenmolecules. The binding energy calculation indicates that theincreased physisorption energy between NH3 molecules andSnS2 may result from the O2 gas facilitating the adsorption ofNH3. Consequently, the sensing mechanism of the SnS2 canhave two routes in terms of the existence of oxygen. In absence ofoxygen, the SnS2 is dissociated, forming S vacancies at hightemperature as shown in eq 5:

→ + +x

SnS (s) SnS(s)1

S (g) Vxx

2 s (5)

where x = 2−8, which represents various S species determinedduring this process, and Vs

x represents neutral sulfur vacancy.In an anaerobic environment, NH3 reacts with the sulfurvacancies in the SnS2 and generates electrons, resulting in adecrease of resistance as shown by eq 6:

+ ↔ + + ++ + −e2NH (ads) V N (g) 6H V 8x3 s 2 s

2(6)

In the presence of oxygen at elevated temperature, oxygenabsorbed on the surface of SnS2 takes electrons from the SnS2,creating a resistance increase. With oxygen involvement, the

Figure 5. (a) Response factor and response time of sensors made of 2D SnS2 flakes in the presence of 10 ppm of NO2 in synthetic air balance as afunction of operation temperatures. (b) Measured cross-talk of 2D SnS2 flakes toward H2 (1%), CH4 (10%), CO2 (10%), H2S (56 ppm), and NO2(10 ppm). (Inset shows the calculated molecule−surface adsorption energies of 2D SnS2 flakes toward the gases together with NH3.) (c) Response−recovery characteristics of the SnS2 based sensor to 100 ppm of NH3 in different background oxygen contents at 200 °C. (Inset shows thecorresponding response curve.)

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response is enhanced because the reaction or gas adsorptionbetween ammonia and oxygen is much easier than that betweenammonia and the sulfur vacancies. It appears that the selectivesorption by defect structure is a considerable factor in additionto their structural and compositional design for sensorapplications of TMDs.

■ GAS SENSING PROPERTIES OF 2DNANOSTRUCTUREMATERIALS ANDMETALOXIDEHYBRIDS

The reported results have demonstrated promising potential of 2DTMDs for gas sensor application, but the experimental results on thesensing performance of the 2D TMDs have not been as high as theirtheoretical values. Such discrepancy can result from the restacking oraggregating of nanosheets due to the electrostatic force between eachpair of layers, which influences a reduced gas response by depleting thenumber of reaction sites. Therefore, strategies to prevent restacking of2D TMDs layers are devised, including the formation of hierarchicalstructures,65,66 incorporation of nanomaterials, and novel metaldoping.67,68 These methods provide improved sensitivity, but thesensors still have a deficiency in selectivity by responding to more thanone analyte gas. A possible approach to improve selectivity is to designan electronic nose, an array of single chips to identify several gasessimultaneously. In a single unit, decoration or doping of novel metalssuch as platinum, palladium, silver, and gold, could be a reasonablesolution to increase the selectivity via catalytic activities. The use ofnovel metals improves gas sensing performance by lowering theactivation energy because of electronic sensitization and spillovereffects.69 Some novel metals have a high affinity for specific gas species.Catalyzing the dissociation of O2 subsequently spills out O

− ions ontothe surface of the metal oxide, which results in an increased responseand faster reaction time.70 However, the cost of novel metal and itscatalytic poisoning largely offset its merit. Not only selectivity andresponse, but also long-term stability of the 2D TMD sensor is anothercritical issue. When 2D TMDs are tightly bound with oxygen moleculesin ambient atmosphere, their sensing properties have not beenmaintained for extended periods of time.13,71 To overcome theproblem, the application of external energy such as thermal energy orUV energy was attempted to eliminate oxygen molecules by breakingtheir bonds.72

As an alternative approach to enhance selectivity and sensitivity foran array, the integration of TMDs with metal oxides can be consideredfor multiple benefits. This combination may solve several challengesincluding sensitivity, selectivity, and stability issues. Based on thematerial features of TMDs and metal oxides, the advantages anddisadvantages of each material for gas sensing applications are listed inTable 3.73

It should be noted that these advantages and disadvantages of eachclassification are not absolute for either category. Rather, they arecomparative features that provide reasonable options when designing agas sensor. With their combination, the agglomeration of TMD nano-sheets can be avoided. The resulting porous structure exposes muchmore surface of TMDs, providing a higher chance to interact with gasmolecules. More importantly, the hybrid nanostructure can enhancethe gas sensing performance by organizing a heterojunction at the inter-face between the two materials with a creation of either an n/p-typenanostructure or an n/n- or p/p-type structure. After building contactbetween two semiconducting materials, the Fermi levels across theinterface will be equilibrated to the same energy state. Charge carrierswill be transferred, and depletion or accumulated layers will be gener-ated at a heterojunction. Thereupon, electrical features of the hybridscan be tuned toward the significant improvement of gas sensingproperties.

The collective benefits of TMDs and metal oxides hybridization canbe broken down into three general aspects: geometrical effects, elec-tronic effects, and chemical effects:22

• Geometrical effects(1) The construction of a hierarchical structure prevents the

agglomeration or restacking of 2D TMD nanosheets ormetal oxides, which enlarges the active surface area forreaction with analytes.

(2) The porous and controlled nanostructure by the incor-poration of metal oxides to 2D TMDs during the syn-thesis facilitates gas absorption and diffusion correspond-ing to fine-tuning of the metal oxide gas response.

• Electronic effects(1) The potential energy barrier and charge carrier depletion

zone at the heterojunction can be modulated by gasadsorption and desorption to act as additional reactionsites.

(2) The built-in internal electric field formed at the interfacepromotes additional oxygen adsorption and charge carrier’sconveyance.

(3) The depletion zone of charge carriers at the hetero-junction acts as a passivation layer to prevent the inter-action between atmospheric oxygen and TMDs.

• Chemical effects(1) The chemical bonds created between 2D TMDs and

metal oxides can act as efficient charge transport bridgesduring the gas sensing process.

(2) The density of chemisorbed oxygen on the surface ofmetal oxides can be increased by the interfacial chemicalbonds.

(3) The receptor function of metal oxides can improveselective adsorption of analytes.

Predicated upon these synergistic effects, the hybrids can be consid-erably developed, but little work has been done regarding gas sensingproperties of TMD/metal oxide hybrids as compiled in Table 4.

A similar effect of hybridization in graphene and metal oxide hybridswas assumed in TMD systems.69,73 Metal oxides such as tin oxide(SnO2), zinc oxide (ZnO), and tungsten oxide (WO3) with variousnanostructures have been incorporated to 2D TMDs mostly by hydro-thermal or layer-by-layer methods. Through the hybridization, theability to measure analyte gases can also be diversified to ethanol, H2,NH3, and NO2. The sensing temperature of the hybrids were variedfrom room temperature to 280 °C as a function of the amount ofinsulating metal oxides. As for the sensing mechanism of the TMDshybrids, the metal oxide nanocrystals generally served as the major gasadsorption centers and the 2D TMDs likely act as conductancechannels with semiconducting behavior. Meanwhile, the charge carrierdepletion layer or an energy barrier created at heterojunctions may offerextra reaction sites. To comprehend the improved performance of thehybridization, the junction effect should be highlighted. Severalexamples of hybrids and the proposed mechanisms for the collectiveeffects will be elaborated in the following sections. Since the synergisticeffect and gas sensing performance of the hybrids are ineluctably

Table 3. Advantages and Disadvantages of TMDs and MetalOxide Sensors

TMDs metal oxide

Advantageshigh electron mobility at lowtemperature

short response time

low energy consumption low costhigh gas response long-term stabilitygood compatibility scalable fabricationmechanical flexibility

Disadvantageslow selectivity low electron mobility at low

temperaturesluggish recovery high operating temperaturerelatively high cost high power consumptionlack of long-term stability low gas responselack of scalable fabrication

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influenced by the choice of material with their innate semiconductingproperties, we have classified this section by the hybrid type.MoS2 + Metal Oxide Hybrids. SnO2 is possibly the most studied

n-type semiconductor for gas sensing applications, and the attempts tointegrate SnO2 withMoS2 can be a logical pathway. Yan et al.

74 reportedthat dispersing SnO2 nanoparticles on MoS2 nanosheets (SnO2@MoS2) improved the gas sensing performance to ethanol. Figure 6a

shows the field emission scanning electron microscopes (FESEM)image of SnO2@MoS2 hybrids where overlapped MoS2 nanosheetscreated a 3D flower-like structure. In comparison to pure SnO2, theoperating temperature of the SnO2@MoS2 hybrids became lowered,but the higher response value was obtained. The highest gas responsewas attained at 280 °C for the hybrids while that of pure SnO2 was at340 °C (Figure 6b). In addition, the selectivity to ethanol was also

Table 4. Literature of Gas Sensing Conditions of TMD/Metal Oxide Hybrids and Their Sensing Performances

TMD MO synthesis processmorphology

of MO

workingtemp(°C) analyte concentration

gasresponse,ΔR/Ra (%)

limit ofdetection effect

dominantsensingparts ref

MoS2 SnO2 hydrothermal NPs 280 ethanol 200 ppm 120a 50 ppm operable MO/p-njunction

74temperature↓gas response↑selectivity↑

hydrothermal nanofibers 230 TEA 100 ppm 24.9a 5 ppm operable MO/p-njunction

75temperature↓gas response↑selectivity↑stability↑

thermal treatment NPs RT NO2 10 ppm 28 0.5 ppm stability↑ MoS2/n-njunction

76gas response↑selectivity↑recovery time↓

ZnO layer-by-layer nanorods RT NH3 100 ppm 61.92 12 ppb gas response↑ MO/n-njunction

77

two-stephydrothermal

NPs 260 ethanol 50 ppm 42.8a 50 ppm gas response↑ MO 78selectivity↑

TiO2 hydrothermal nanotubes 150 °C ethanol 100 ppm 14.2a 50 ppm gas response↑ MoS2/p-njunction

79

Co3O4 layer-by-layer nanorods RT NH3 0.1 ppm 10.3 0.1 ppm gas response↑ MO/p-njunction

80selectivity↑response andrecoverytime↓

WS2 TiO2 vacuum filtering quantumdots

RT NH3 500 ppm 56.69 20 ppm gas response↑ MO/p-njunction

81selectivity↑response andrecoverytime↓

stability↑

WO3 annealing flat shape 150 H2 10 ppm 2.9 1 ppm stability↑ WS2 82NH3 4.9 1 ppmNO2 100 ppb

mix NPs 25−250 NH3 2000 ppm 12.6b 250 ppm operable MO/n-njunction

83temperature↓gas response↑selectivity↑

SnS2 SnO2 oxidizing NPs 80 NO2 8 ppm 5.3a 1 ppm gas response↑ SnS2/n-njunction

84response andrecoverytime↓

microwave/thermaloxidation

NPs 100 NO2 1 ppm 51.1a 125 ppb gas response↑ SnS2/MO/n-njunction

66selectivity↑

thermal oxidation NPs RT NH3 10 ppm 1.16 10 ppm gas response↑ MO/n-njunction

85

hydrothermalsulfuration

nanotubes RT NH3 100 ppm 2.48c 1 ppm gas response↑ n-njunction

86

aRa/Rg.b250 °C. cIg/Ia. Abbreviations: MO, metal oxide; NPs, nanoparticles; RT, room temperature.

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highly improved through the hybridization (Figure 6c). It is reasonableto assume that the enhanced sensing performances result from thecollective geometrical and electronic effects. The increased surface areaof MoS2 layers by inserted SnO2 nanoparticles offers more active sitesfor better facilitation of ethanol diffusion and faster electron transfer.In addition, the p-n junction at the interface of SnO2 (n-type) andMoS2(p-type) also forms a built-in internal electric field, which acceleratesthe electron transfer. The tuned heterojunctions of MoS2 and SnO2 wasalso reported by Qiao et al.75 for the efficient gas sensor. In their work,MoS2 (n-type)/SnO2 (p-type) heterojunctions were tailored as afunction of MoS2 loading, and this hierarchical p-n heterojunctionexhibit great potential for both gas sensor and catalysts. With a smallamount of MoS2 (molar ratio of Mo/Sn = 0.53), the p-n junctionshowed superior sensing selectivity and long-term stability totrimethylamine (TEA) at 230 °C. Such superior sensing performanceof the hybridized sensor based on TMDs and metal oxide was alsocommonly observed at low temperature. Cui et al.76 decorated SnO2nanocrystals onto MoS2 nanosheets by a wet chemistry methodfollowed by an annealing process and reported an interesting hybrid-ization effect on sensing performance at room temperature. Theoctahedral phase (1T-MoS2) with metallic properties was transformedto trigonal prismatic MoS2 (2H-MoS2) with semiconducting propertiesafter annealing at 300 °C for 1 h. The combination of the pristine2H-MoS2 with n-type semiconducting properties and absorbed oxygenacting as p-type dopant resulted in bipolar characteristics afterhybridization. The gas sensing behavior of the SnO2 decorated - MoS2was surprisingly converted to p-type toward NO2 gas, while both SnO2and MoS2 showed n-type behavior (Figure 6d). The reasons for thephenomena are unclear. However, the authors believed that the SnO2nanoparticles served as strong p-type dopants to the MoS2 nanosheets,which induces dominant p-type behavior of the nanohybrid. Theirproposed sensing mechanism predicts that SnO2 nanocrystals likelyserved as the major gas adsorption centers and MoS2 acted as theconductance channels at room temperature. Intimate electrical contactbetween two dissimilar semiconducting materials usually results incharge transfer and the formation of a charge depletion layer. Since thework function of the SnO2 (5.7 eV) is larger than that of MoS2 (5.2 eV),electrons are transferred from MoS2 to SnO2. This transfer createsdepletion layers and a Schottky barrier (Figure 6e). The createddepletion layers at the interface can prolong the stability and improve

selectivity of the hybrid sensor (Figure 6f). Because the electron deplet-ion zone acts as a passivation layer, the impeded interaction betweenatmospheric oxygen and MoS2 increases the long-term stability of theSnO2-decorated MoS2 nanohybrids. This explanation was validated bythe experimental results that show extended stability with increasing theamounts of SnO2 loaded. In addition, the improved selectivity to NO2can be ascribed to the incorporation of SnO2 because the pristine MoS2was sensitive to both NO2 and NH3.

87,88 However, the mechanism onthe selective response of SnO2 to NH3 remains to be investigated.

ZnO is another n-type semiconductor with a wide band gap of 3.37 eV.ZnO has been intensively investigated as a sensing material alongwith SnO2. The gas sensing characteristics of MoS2 and ZnO hybridswere explored by Zhang et al.77 A multilayered film of MoS2/ZnOwas prepared via a layer-by-layer self-assembly technique withpoly(diallyldimethylammonium chloride) (PDDA)/poly(sodium4-styrenesulfonate) (PSS) as precursor layers (Figure 7a). Gas sensingproperties of MoS2/ZnO, MoS2/PDDA and a pure ZnO sensor werecomparatively studied. During ammonia sensing, the MoS2/ZnOhybrid presented n-type sensing behavior at room temperature. Thehigher gas response was observed from the MoS2/ZnO sensor in com-parison withMoS2/PDDA or pure ZnO sensor (Figure 7b). For example,the normalized gas response values to 5 ppm ammonia are 23.96%,19.63% and 5.97% for the MoS2/ZnO, MoS2/PDDA, and pure ZnOsensors in order. In addition, the fast response and recovery time of theMoS2/ZnO sensor were measured to be 10 and 11 s under 50 ppmammonia. The enhanced gas sensing performance was explained basedon the synergistic effects of both MoS2 and ZnO. When oxygen isabsorbed on the ZnO surface mainly in the form of O2

− at roomtemperature, the oxygen ion acts as an electron acceptor and captureselectrons from the conduction band of the sensing material. Theabsorbed oxygen ions react with gas molecules and transfer electrons tothe conduction band of ZnO and MoS2/ZnO interface. An energybarrier (ΔEB =WZnO−WMoS2) can be created at the interface ofMoS2 and

ZnOwhereWZnO = 4.43 eV andWMoS2 = 4.33 eV. Figure 7c illustrates theband structure of MoS2/ZnO upon the creation of a heterojunction atthe interface after equilibrating the Fermi energy level. Due toaforementioned transferred electrons, the Fermi level of ZnO shiftstoward the conduction band, and the energy barrier height at theinterface of MoS2/ZnO is reduced. As a result, the resistance of the

Figure 6. (a) FESEM image of SnO2@MoS2 composites. (b) Correlation between gas response to 200 ppm ethanol and the operating temperature forthe sensors based on SnO2@MoS2 composites, and pure SnO2 nanoparticles. (c) Response of the sensor based on SnO2@MoS2 composites to varioustest gases. (d) The room temperature dynamic sensing response of MoS2 nanosheets with and without SnO2 nanocrystal decoration to 10 ppm of NO2in a dry air environment, indicating the SnO2 nanocrystals significantly enhanced the stability of MoS2 in the dry air. (e) Band diagram of the MoS2/SnO2 nanohybrid. The EF,SnO2

and EF,MoS2 are Fermi levels of SnO2 andMoS2, respectively. The CB and VB are the conductance and valence band edgesof MoS2, respectively. d is the thickness of the electron depletion zone, andΦB is the Schottky barrier height. (f) Sensing response of the MoS2 /SnO2nanohybrid to various gases at a concentration of 10 ppm.

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MoS2/ZnO hybrid sensors decreases. Along with the n-n junction effect, ahigher specific surface area of the hybrid structure can provide higherquantities of efficient electron pathways and active centers of gas absorption.In contrast to n-type semiconducting oxides, Zhang et al.80 hybrid-

ized a p-type semiconductor cobalt oxide (Co3O4) with MoS2 toinvestigate the function of opposite majority carriers in the hybrid. TheMoS2/Co3O4 hybrid film with five self-assembled layers yielded thehighest gas response among the one, three, five, and seven layers,probably due to a good balance between conductivity of the sensor andgas diffusion between layers. The fabricated p-n-type hybrids showedp-type sensing behavior toward ammonia (Figure 7d). In addition, theselectivity of the MoS2/Co3O4 hybrid toward ammonia was superior toethanol (CH3CH2OH), benzene (C6H6), formaldehyde (HCHO), andacetylene (C2H2) (Figure 7e). In this hybrid, Co3O4 serves as an elec-tron acceptor, creating a hole accumulation layer (HAL). Underammonia exposure, the gas molecules react with adsorbed oxygen ions(Oads

2−) by releasing electrons from ammonia, resulting in thedecreased HAL width and increased resistance of the hybrid. It isbelieved that the results of the MoS2/Co3O4 hybrid can provideconvincing evidence on the junction effects in sensing mechanisms ofTMD-MO system. MoS2 is an n-type semiconductor with 1.2−1.9 eVband gap while Co3O4 is a p-type semiconductor with 2.07 eV band gap.With their contact, the interdiffusion of each majority carrier drives toform a depletion layer at the interface (p-n junction). The holes inCo3O4 and electrons in MoS2 create a self-built electric field andestablish a depletion layer at the p-n junction that is equilibrating withthe Fermi level. The depletion layer can be modulated as a function ofadsorption and desorption of the gas molecules. Figure 7f depicts thevariation of the width of the depletion layer upon exposure to air andammonia gas. With ammonia injection, free electrons were releasedfrom ammonia through the interaction between Oads

2−, and neutralizedthe holes in the Co3O4. Accordingly, it contributes to the expansion ofthe depletion layer, which increases the resistance of the hybrid sensor.Such combination of the junction effect along with the electrical andstructural benefits from its layered structure can offer an understandingof superior sensing performance of the MoS2/Co3O4 hybrid.WS2 + Metal Oxide Hybrids. TiO2 is a well-known wide band gap

n-type semiconductor with good thermal and chemical stability. The

sensing properties of TiO2 quantum dot (QD) decorated 2DWS2 weredemonstrated by Qin et al.81 Chemically exfoliated 2DWS2 nanosheetswere hybridized with TiO2 QDs (3−4 nm) under different ratios from0.44 to 1.65 of TiO2/WS2 by vacuum filtering. The gas response toammonia was highest with 0.44 TiO2/WS2 ratio and decreased withfurther increasing TiO2 amount (Figure 8a). Excessive TiO2 QDscoverage appears to degrade the electrical conduction in addition to thedifficult interaction between the ammonia gas and the hybrid. Thehybridized TiO2/WS2 sensors had higher gas response and selectivity,with shorter recovery time and improved stability compared to bareWS2 monolayer-based sensors. Figure 8b presents nearly 17 timeshigher response of the TiO2/WS2 hybrid than the bare WS2 withimproved selectivity to ammonia, as shown in Figure 8c. Aftercombining two materials, the electrons in the conduction band of WS2were transferred to that of TiO2, followed by the creation of a largedepletion region at the interface between TiO2 QDs and WS2, asillustrated in Figure 8d. The improved gas response of the hybrid can beascribed to the expansion of depletion layers and the numbers ofreaction active sites by TiO2 QDs. Limited improvement of sensingperformance in the large size of TiO2 for the hybrids implies that thesize of the metal oxide should be carefully chosen. Interestingly, thehybrids showed improved stability on gas sensing under ambientconditions. It is reported that absorption of oxygen from atmosphereonto the surface of 2Dmaterials induces to the negative shift of the basecurrent of the WS2 sensor at room temperature, influencing on long-term stability.89 In the hybrids, the presence of the depletion region onthe interface would reduce the interaction between WS2 and ambientoxygen gas. The depletion region could function as passivation layers,resulting in significantly extended stability of the hybrids.

SnS2 +Metal Oxide Hybrids. SnO2 can be an excellent material tobe grafted to SnS2 because the simple physical mixing between SnO2and SnS2 in this hybrid can leverage to the composition-tunable bandgap. Besides the heterojunction effect, the SnS2/SnO2 hybrids havedemonstrated to create a hierarchical structure for better geometricaleffects. Flower-like hierarchical SnS2/SnO2 hybrids were fabricated by amicrowave method and thermal oxidation by Hao et al.66 The averagesize of the flower-like SnS2/SnO2 nanohybrids is around 1 μmdiameter,with a uniform distribution of ultrafine SnO2 nanoparticles (Figure 9a).

Figure 7. (a) SEM characterization of MoS2/ZnO nanocomposite (b) Normalized responses of MoS2/ZnO, MoS2/PDDA and ZnO film sensorsexposed to various ammonia gas concentrations at room temperature, and the inset indicates the response and recovery characteristics of these sensorsexposed to 50 ppm ammonia gas. (c) Schematic illustration of the energy-band structure of the sensor (E0 is vacuum-energy level,W is work function,Eg is energy band gap, EF is Fermi-energy level, EC is the conduction band, and EV is the valence band). (d) Normalized response of the MoS2/Co3O4,MoS2, and Co3O4 film sensors toward various ammonia concentrations. (e) Selectivity of the MoS2/Co3O4 film sensor upon exposure to ammonia(NH3), ethanol (CH3CH2OH), benzene (C6H6), formaldehyde (HCHO), and acetylene (C2H2) with concentrations of 5 and 2 ppm, respectively.(f) Energy band structure diagram and resistance variation for the n-type MoS2/p-type Co3O4 hybrid in air and ammonia.

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All SnS2, SnO2, and SnS2/SnO2 hybrids showed n-type sensingbehavior to NO2 gas. From the experiment to form SnO2 on SnS2 bypartial oxidation, the gas response was enhanced with increasingoxidation time, and then the response declined after 1 h oxidation. Theinitial increase can be attributed to the facilitated adsorption of NO2 bythe formed SnO2 nanoparticles. Excessive oxidation to fully oxidize

SnS2 nanosheets degraded the sensing response (Figure 9b). Thesensing mechanism of SnS2/SnO2 nanohybrids can be understood interms of two factors: heterojunction and hybridized nanostructure(Figure 9c). Electronically, the n-n junction is constructed betweenSnS2 nanosheets and SnO2 nanoparticles. With band bending, anaccumulation layer is built on the SnO2 side, and a depletion layer is

Figure 8. (a) Response of TiO2 QDs/WS2 nanohybrids and the current baseline as a function of different molar ratios (0.22, 0.33, 0.44, 0.88, 1.65)upon exposure to 250 ppm of NH3. (b) The response of gas sensors based on 0.44 TiO2 QDs/WS2 nanohybrids, large particle TiO2/WS2 andmonolayer WS2 as a function of different concentrations of ammonia gas from 20 to 500 ppm. (c) Selectivity of the sensors based on the 0.44 TiO2QDs/WS2 nanohybrids and monolayer WS2 upon exposure to 500 ppm of various gases. (d) Energy band diagram of the TiO2/WS2 nanohybrids andthe schematic of the interfacial interaction between the TiO2/WS2 nanohybrids structure and NH3 molecules.

Figure 9. (a) SEM image of the fabrication of SnS2/SnO2 nanocomposites. (b) Response curves of the sensors based on pure SnS2, SnS2−SnO2, andpure SnO2 with different oxidation times toward 1 ppm of NO2 at an operating temperature of 100 °C. (c) Band structure model of the SnS2/SnO2heterojunctions, EF is the Fermi level, and EC and EV are the conductance and valence bands, respectively. (d) Semiquantitative analysis of the fractionof O−Sn−S among the total of O−Sn−O, O−Sn−S, and O−Sn. (e) Dynamic response−recovery curves of the obtained samples to 100 ppm of NH3at room temperature. The inset shows a larger version of SnS2. (f) Schematic of the SnO2−SnS2 hybrid with [a] a small amount and [b] a large amountof chemical bonds at the interface and their corresponding gas-sensing process.

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formed on the SnS2 side. Under oxygen presence, the accumulationlayer can be shrunk by oxygen absorption followed by the formation ofO2

− by taking electrons from the layer. Such an accumulation layerresults in the decreased resistance and increased charge transfer duringthe sensing process. In contrast, the NO2 gas molecules react with O2

−

by withdrawing electrons from SnO2. Then more electrons are trans-ferred from SnS2 to SnO2 until a new equilibrium is achieved, leading toa widened depletion layer and increased resistance. As a result, it isevident that the number of n-n junctions significantly influences thesensing response. Structurally, the large space between SnS2 nanosheetsin the hierarchical structure via dispersed SnO2 is beneficial for the gasmolecules diffusion. Chemically, the bonds of S−Sn−O createdbetween SnS2 and SnO2 act as an efficient charge transport bridgeduring gas sensing process. The chemical effects were supported bysimilar experiments of SnO2−SnS2 hybrids by thermal oxidation of SnS2at 300 °C.85 XPS results confirmed that the peak intensity of interfacialbonds increased due to the increased amount of interfacial bondsbetween SnO2 and SnS2, and then it decreased after 1 h of oxidation(Figure 9d). The highest response to ammonia was attained from theSnO2−SnS2 hybrid sample oxidized for 1 h (Figure 9e). It is apparentthat the interfacial bond of SnO2 and SnS2 is closely related to thesensing mechanism. At the interface, the charge carriers can be trappeddue to dangling bonds, and an additional depletion layer can be formedat the interface. Equation 7 explains the density of chemisorbed oxygenon the surface of SnO2 governs the gas sensing properties of the SnO2−SnS2 hybrids.

i

kjjjjjj

y

{zzzzzzε

= −Q r NeQkTN

3 exp22 2 d1

i2

d1 (7)

where Q2 is the density of chemisorbed oxygen on the surface of SnO2,r2 is the radius of SnO2 hemispheres,Nd1 is the donor density of SnS2,Qiis the density of chemisorbed oxygen on the interface state density, ε isthe dielectric constant of SnS2, k is Boltzmann constant, T is tem-perature. With a small number of interfacial bonds, few electrons areable to pass across the depletion layer because of the high energybarrier. According to eq 7, a small amount of chemisorbed oxygen canbe generated, which results in a low gas response. On the other hand,the density of chemisorbed oxygen can be enlarged with the formationof interfacial bonds for the enhanced gas performance (Figure 9f).Therefore, various parameters associated with hybridization should beconsidered to predict the overall gas response.

■ CONCLUSIONS AND OUTLOOKIn this Review, we comprehensively compiled achievements inrecent research on 2D TMDs and metal oxide hybrids. Thestructure and basic characteristics of newly emerged 2D TMDswere introduced with a brief explanation on the sensing mech-anisms of metal oxide and 2D TMDs. Their synthesis methodsand distinctive gas sensing properties were subsequently sum-marized. To improve and refine the sensing performance of 2DTMDs, the integration of 2D TMDs with metal oxides wasexplored. The collective benefits and mechanisms of TMDs andmetal oxides were discussed in three aspects: geometrical effects,electronic effects, and chemical effects. Some results showedpromising performances such as high sensitivity and selectivityto analyte gases even at room temperature. Such achievementsoffer great potential for practical implementation as gas sensors.Although significant progress has been demonstrated, there

are still remaining questions to understand the underlyingmechanisms to develop devices for real-life applications. First, aclear understanding of the synergistic or hybridized effects in 2DTMD and metal oxide hybrids should be determined. Thegeometrical effects of hybridization have been often described,but the electronic and chemical effects have not been clearlyexplained. Throughout relevant research papers, there arevarious electronic and chemical explanations for the improved

gas sensing performance. However, there is no clear answer as towhich electrical modification corresponds to each specificperformance improvement. Most references only addressedthe charge carrier flow based on the Fermi energy difference andthe formation of depletion layers regardless of the type ofheterostructure. The relationship between the modified proper-ties and the enhanced performance should be more clearlydefined in terms of electronic or chemical mechanisms. In addi-tion, little work has been done to address a thorough scientificcomprehension of enhanced selectivity due to the hetero-structures. Several references mentioned improved selectivitythrough hybridization, but such improvement has not beenclearly explained. More simulated or experimental results canclarify this vague explanation. Second, a rational justificationof the hybrid design is required in terms of material selection.When designing a hybrid, a researcher should consider theFermi energy and work functions of each material, as well as thedominant sensing material. The direction of the charge carriertransfer at the interface and the dominant sensing materialwill be critical in the sensing mechanism and the sensingperformance. Unclear information of the contact type, eitherSchottky or ohmic, at the heterojunction limits the ability todetermine which contact is more favorable to improve the gassensing performance. Further study should be focused on thisaspect.In terms of future prospectives, there are tremendous oppor-

tunities to investigate 2D TMD materials and their boundlesscombinations with metal oxides for sensor applications. Goingforward, new materials, and their hybrids, with innovativedesigns will be a promising option. Compared to graphene-based gas sensors, little attention has been paid to 2D TMDsensors. In addition, the functionalization of TMDs thoughmetal oxide hybridizations is still at an early stage of research.In the case of MoSe2 and MoTe2 for sensor applications, noresults have been published on their possible metal oxidehybrids. In this regard, simulation results could guide the insightfor a heterostructure design prior to an experimental investigation.In addition, an investigation on surface treatment can elucidatethe sensing mechanisms and garnish sensing performance forfurther advancement. Previous studies have examined thesurface treatment of 2D nanomaterials, using techniques suchas UV treatment or thermal annealing. By introducing photo-reactive materials into the heterostructure, the performances ongas response and the response/recovery time can be improved inaddition to gaining knowledge on photoinduced chargertransfer. For instance, it has been reported that the perovskitecrystal structure shows great photoreactivity.90,91 By deco-rating perovskite structured metal oxides on 2D TMDs, photo-activated effects can be magnified, resulting in boostedperformance.

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: dkim@auburn.edu. Tel.: 1-334-844-4864. Fax: 1-334-844-3400.*E-mail: benedicto@gachon.ac.kr. Tel.: 82-011-9766-5558.

ORCIDEunji Lee: 0000-0001-5046-3060Dong-Joo Kim: 0000-0001-5013-4005NotesThe authors declare no competing financial interest.

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■ ACKNOWLEDGMENTSThis research was partially supported by the Korea Institute ofEnergy Technology Evaluation and Planning (KETEP), by agrant funded by the Korea Government Ministry of Trade,Industry and Energy (20158520000210), and by the Agency forDefense Development (ADD) as global cooperative research forhigh performance and lightweight biourine-based fuel cell(UD160050BD). The authors deeply appreciate BenjaminKeith for his valuable efforts on this manuscript.

■ VOCABULARY

gas response, a change of measured signal per analyteconcentration unit; selectivity, the ability of gas sensors toidentify a specific gas among a gas mixture (response of target/Rof interference); stability, the ability of a sensor to providereproducible results for a certain period of time; heterojunction,the interface that occurs between two layers of dissimilarcrystalline semiconductors; Fermi energy, the highest energythat the electrons assume at T = 0 K

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